1. Field of the Invention
The present invention relates to display controllers, and more particularly, to a display controller capable of accessing an external memory for gray scale modulation data.
2. Description of the Related Art
One commonly used type of display panel is a liquid crystal display (LCD) panel. An LCD display panel is a rectangular grid of rectangular or square dots. Acting as a thin double-paned window, the LCD grid is actually transparent electrodes laid out in horizontal rows on one thin pane, and in vertical columns on the other. The liquid crystal formula trapped in between the panes reacts to an electrical field applied to each electrode in the rows and columns. This reaction rotates the polarization of light transmitted through the LCD display. Polarizing layers outside the panes cause the dots to appear light or dark as the polarization changes. There are small gaps between the rows and columns, giving each dot a clear definition.
The display is controlled by continuously feeding dot data to the display. The data is organized into individual pixels, rows of pixels, and full-page frames. Pixels are the individual data dots or bits. These bits are put together into rows. A set of rows makes up a frame. A frame is one full page of the display. LCD data is continuously sent to the LCD panel to refresh the display frame.
Since most LCD displays have no on-board frame buffer memory, the display data must be continuously refreshed. To get a stable, flicker-free image, the display data is sent to the panel at a frame refresh rate (referred to herein as the "frame rate") which falls within a range normally specified by the LCD panel manufacturer. An LCD panel manufacturer may specify, for example, that best results are obtained, i.e., a stable, flicker-free image, when the display data is sent to the panel 60 to 70 times per second, or 60 Hz to 70 Hz.
An LCD controller is typically used to coordinate the transfer of display data to an LCD panel. Two important functions performed by an LCD controller are: 1) gray scale modulation, and 2) sending display data to the display panel within the specified frame rate range.
In order to create an image on an LCD screen, each pixel is constantly being refreshed at the frame rate. If only two different colors are needed, i.e., on (white or bright) and off (black or dark), a zero is always sent for white and a one is always sent for black. For example, assuming that each pixel is refreshed 60 times per second, i.e., a frame rate of 60 Hz, if a pixel is white, the value of zero will be sent 60 times for each second (for that bit), and if the pixel is black (or dark), a one will be sent for 60 times. In this scenario the graphics data (the one and zeros indicating white and black) can basically be fed directly to the display.
However, when more than two colors are needed on the LCD screen, gray scale modulation is performed to create an LCD image that appears to be stable and appears to be some shade between on (white or bright) and off (black or dark). Gray scale modulation is a process of sending a value of one to the screen for a percentage of the time to create a pixel that is light or dark gray. The rate at which the pixels are turned on and off determines how light or dark they appear. For example, if a one is sent for 45 times, and a zero is sent for 15 times (during the 60 Hz refresh), a dark gray will appear on the screen. If a one is sent for 15 times, and a zero is sent for 45 times, a light gray will appear.
In general, an LCD controller receives graphics data and then generates and provides the appropriate ones and zeros to the display panel which are needed to display the specified shade of gray for each pixel in the frame. Because of the nature of LCD displays, gray scale modulation is done in a temporal (or time) and spatial modulated way. The term "temporal" refers to the frequency at which individual pixels are turned on and off. The term "spatial" refers to the relationship of one pixel to an adjacent or nearby pixel. Specifically, in order to prevent flickering, adjacent pixels of the same gray value will be modulated at different frequencies. Thus, the brightness of each pixel in the display is determined by the temporal modulation of the applied voltage pulses to the respective pixels.
Previous LCD controllers performed such temporal modulation by manipulating the graphics data with a fixed algorithm. One available LCD controller uses another set of data, referred to herein generally as gray scale modulation data, in conjunction with the graphics data to perform temporal modulation. That LCD controller includes two on-board registers for holding the gray scale modulation data. The gray scale modulation data is initially supplied to the LCD controller by a CPU. When more gray scale modulation data is needed, the LCD controller interrupts the CPU so it can update the on-board registers. Numerous interrupts, however, reduces the efficiency of the CPU. The number of interrupts can be reduced by increasing the number of registers in the LCD controller that are used to hold gray scale modulation data. However, this has the undesired effect of increasing the LCD controller die size.
Thus, there is a need for an LCD controller which has access to an increased quantity of gray scale modulation data in order to decrease CPU interrupts, but which does not have an increased die size.